1. Field of the Invention
The present invention relates to a gas sensor; and, more particularly, to a micro gas sensor for measuring a gas concentration, and a method for manufacturing the same.
This work was supported by the IT R & D program of the MIC/IITA [2006-S-007-01, “Ubiquitous Health Monitoring Module and System Development”].
2. Description of Related Art
As interests on the environment for the future increase, development of a miniature sensor is increasingly demanded, which can obtain precise and various information within a short period of time. Particularly, to form a pleasant housing environment, cope with harmful industrial environments, and manage food materials and grocery producing processes, efforts are being made to achieve miniaturization, high precision and low price of a gas sensor that facilitates measuring of a concentration of associated gases.
Currently, due to the development of a semiconductor manufacturing technology, the gas sensor is gradually evolving from a typical ceramic sintering or a thick film type structure into a microelectromechanical (MEMS) micro gas sensor.
A measuring method of the micro gas sensor, which is most widely used, is to measure electrical-characteristic changes of a gas sensitive layer when a gas is absorbed to the gas sensitive layer. In general, metal oxide such as SnO2 is used for the gas sensitive layer, and changes in electrical conductivity according to a concentration of a target gas are measured, which is relatively simple.
When the gas sensitive layer of metal oxide is heated to a high temperature, the changes in a measured value are more notable. Accordingly, temperature control is necessary for fast and precise measurement of the gas concentration. Also, before the gas concentration is measured, gas species and moisture absorbed to the gas sensitive layer are removed by heating to a high temperature so as to reset the gas sensitive layer to an initial state.
In the gas sensor, temperature characteristic directly affects critical measurement factors of the gas sensor such as measurement sensitivity, reset time, and reaction time of the sensor. Thus, a micro heater is effective for efficient heating, which locally and uniformly heats only the gas sensitive layer.
However, in the case of the micro gas sensor, if a large amount of power is consumed in controlling a temperature, a large battery or power supply source is required, and thus the entire size of a measuring system is increased even if the volume of the sensor and a measuring circuit is small. For this reason, to implement the micro gas sensor, a structure resulting in low power consumption must be considered primarily.
In most known manufacturing processes for the micro gas sensor, a silicon substrate having very high thermal conductivity is mainly used. In order to reduce heat loss, an etched pit or a groove is formed in a sensor structure through a bulk micromachining process to form a structure such as a membrane, a cantilever, or a bridge suspended from the substrate, and then a micro heater, an insulation layer, and a gas sensitive layer are sequentially formed on the structure.
However, the structure such as the membrane, the cantilever, or the bridge cannot remove the air existing within the structure, and thus there is a limitation in reducing the heat loss. Also, since the micro gas sensor is formed mainly through substrate etching, there is a limitation in miniaturizing a sensor device, and it is difficult to apply a standard complementary metal oxide semiconductor (CMOS).
Also, even if the micro gas sensor having the afore-mentioned structure has a suspended structure, the micro gas sensor includes an open cavity opened in one direction and having a large height difference. This causes inflow of dust particles or disturbs flow around the sensor due to the large height difference, and thus a measured value becomes inaccurate.
For commercialization, the micro gas sensor must be reliably driven for about two to three years. This condition is very strict for the gas sensor that undergoes repetitive heating and cooling within a temperature range between approximately 100° C. and 600° C. by the micro heater. An issue in this repetitive heating and cooling process is damaged by thermal stress from a temperature gradient applied to a sensor structure suspended from the substrate and mechanical impact.
In a conventional method to solve this issue, a base support layer of the suspended structure is formed as a single layer of silicon oxide (SiO2) or silicon nitride (Si3N4), or a stacked layer or multiple layers of three or more layers having different thicknesses, such as Si3N4/SiO2, SiO2/Si3N4, Si3N4/SiO2/Si3N4, or SiO2/Si3N4/SiO2 to achieve stress balance. Then, a micro heater, a sensing electrode, and a gas sensitive layer are sequentially patterned thereon to improve structural brittleness.
However, even if the stress balance is achieved, structural safety cannot be ensured when the temperature non-uniformity increases on the heated suspended structure because those materials are insulator materials having high brittleness and low thermal conductivity. Accordingly, the durability of the micro gas sensor is associated with the constituent material of the structure, and a design of the micro heater.
As mentioned above, although many researches have been conducted on the micro gas sensor, the conventional micro gas sensor still needs to be improved with regard to thermal insulation, power consumption, temperature uniformity, measurement precision, durability, and size.